We propose to determine the quantitative relationships between the cellular oxygen concentration and the cellular metabolic state. The mitochondrial are responsible for most of the cellular oxygen consumption, utilizing the oxygen and cellular reducing power (substrates) to produce the require adenosine triphosphate as the energy supply. A mathematical description of these interrelationships has been developed which will be tested for its ability to fit the mitochondrial behavior both in isolated suspensions and in intact cells. This behavior will be tested for a wide range of oxygen concentrations and substrates in order to provide a basis for understanding tissue behaviour under low oxygen conditions. The properties and mechanism of the enzyme which directly reacts with molecular oxygen will also be examined in order to understand its central role in the control of the mitochondrial (and, therefore cellular) respiratory rate.